1. Field
The embodiments described herein relate to, in general, a communication system. More particularly, some embodiments concern a wireless communication system using adaptive hopping sequences in which a network coordinator monitors the communication among communication devices in the network and adapts the hopping sequence of the network to minimize the chance that communication is disrupted.
2. Description
A number of environments, including industrial automation applications, may require a highly reliable network for communication. In some applications if several consecutive packets from a controller to/from an input/output (I/O) device (such as a motor switch) are lost or delayed beyond a certain threshold, the automation system is disturbed. Unfortunately, in many areas where an automation network is deployed (e.g., factory halls, train stations, etc.), the RF spectrum is often filled with interference. As a result, devices using traditional wireless technologies such as IEEE 802.11a/b/g/n running on a single frequency may be disrupted. As a result, such wireless technologies are not suitable for direct use in industrial automation applications.
Some previous systems have tried to solve the problem of finding a current optimal frequency (or frequencies) for a communication system. Such prior attempts may be generally classified into the two categories of Dynamic Frequency Selection (DFS) and Adaptive Frequency Hopping (AFH).
Using DFS, all devices in a network cell use a primary frequency for a period of time until the quality of the frequency has deteriorated. At that moment, the entire cell moves to an alternative frequency. The alternative frequency may be announced either before a frequency change is necessary (i.e., proactive) or after it becomes necessary (i.e., reactive).
For reactive DFS systems, a major disadvantage is that when the primary frequency suffers from interference, it becomes difficult or even impossible to announce the alternative frequency reliably such that all devices in the network become aware of the new frequency. Proactive DFS systems are more reliable than reactive ones. However, right after the primary frequency has failed and before a replacement alternative frequency is chosen and made known to all devices in the network, if the alternative frequency also fails, then communication is interrupted. Therefore, DFS systems are fundamentally not highly resilient to interference.
Using AFH, each communication device continuously hops over a number of frequencies. Once a frequency is detected as being unsuitable for further use, the unsuitable frequency will be replaced with another frequency whenever it appears in the hopping sequence. This process is known as frequency re-mapping.
AFH systems may be more resilient to interference because the hopping sequence consists of hops over a number of frequencies. Therefore, unless all of the frequencies in the hopping sequence are impaired by interference, the devices can still communicate with each other, albeit at a much lower throughput and at a higher delay, until the bad frequencies have been remapped to good frequencies. A problem with using AFH for industrial automation (and certain other) applications is that such applications cannot tolerate consecutive packets with a high delay or packet losses. However, frequency remapping is not instantaneous since the clients must first be notified of the new frequency map. To illustrate, consider a simple example where there are 3 frequencies available for frequency hopping, where the current hopping sequence is 1, 2, 3, 1, 2, 3, . . . for time slots 1, 2, 3, 4, 5, and 6, respectively. In this example, the hopping sequence repeats every 3 slots even though 6 time slots are shown here. When frequency 1 is determined to be unusable due to interference, it may be replaced with either frequencies 2 or 3. Assuming that frequency 1 is remapped to frequency 2, the new hopping sequence after adaptation is 2, 2, 3, 2, 2, 3, . . . for time slots 1 through 6, respectively. If at this moment, frequency 2 also suffers from interference, the system will suffer 2 back-to-back hops from time slot 1 to 2, and from 4 to 5. This type of scenario is not ideal as industrial automation (and other) applications may be sensitive to consecutive losses and delays. Preferably, the adapted hopping sequence should have been 2, 3, 2, 3, 2, 3 . . . after the first failure. In such a case, even if one of frequency 2 or 3 were to immediately fail, there would not be 2 consecutive back-to-back bad slots. It is noted that one cannot possibly remap frequencies 1, 2, 3 to achieve the hopping sequence of 2, 3, 2, 3, 2, 3, etc.
As illustrated by the proceeding discussion and brief example, it is seen that the design of the new hopping sequence is an important consideration in keeping a frequency hopping communication system or process operating without interruption.